This invention relates to sensitive, fast responding fluid measuring apparatus and, more particularly, to an improved hot wire anemometer.
In a hot wire anemometer, the hot wire is connected to serve as one arm of an electrical bridge circuit. Current passing through the hot wire heats the wire, thereby increasing its resistance. The hot wire is disposed in an elongated cavity through which the gas to be measured flows and cools the hot wire accordingly. If the type of gas passing through the cavity is known, the resistance change of the hot wire is a measure of the gas flow rate. If the flow rate of the gas passing through the cavity is known, the resistance change of the hot wire is a measure of the thermal conductivity of the gas and, hence, the gas type.
My U.S. Pat. No. 3,971,247, which issued July 27, 1976, discloses a thin elongated hot wire bent in half to extend along the length of a cavity formed in a housing. The ends of the hot wire are soldered to pads on a printed circuit board located at one end of the cavity for support and electrical connection to a bridge circuit. The middle of the hot wire is wrapped around a rod at the other end of the cavity for support. The rod is deformed to exert tension on the hot wire as its length changes. Thus, for a cavity having a given length, the length of the hot wire can be doubled and a corresponding increase in sensitivity can be achieved. But, the probability of a short circuit by contact between halves of the hot wire or the hot wire and the sides of the cavity rises, as the length of the cavity increases.